General practice in potash ore flotation aims at maximum recovery of coarse sylvite (KCl) particles. Depending on sylvite grain size, the potash ores are ground to either -6 mesh (3.36 mm) or -8 mesh (2.33 mm) and are classified into +20 mesh (0.85 mm) coarse and -20 mesh fine streams. In order to achieve maximum recovery of the coarse particles, these two streams are reagentized separately and then are usually floated together in mechanical flotation cells. Owing to the intense stirring and turbulence in mechanical cells, flotation recoveries of coarse sylvite particles are commonly low (around 60%), and slime release is severe due to attrition and breakage of the coarse particles.
Flotation columns, which belong to a family of pneumatic flotation machines, have been widely applied in the flotation of fine mineral particles. Wash water supplied to the froth at the top of the column is commonly used to clean the froth products. Columns were also shown to perform better in the flotation of coarse particles [J. Laskowski & M. Marchewicz, Przeglad Gorniczy, 25:438-441 (1969)]. Nearly quiescent conditions in a column provide an ideal environment for coarse particle flotation. Upward pulp flow, co-current to the rising bubbles, was shown to further improve the flotation of coarse particles by assisting in the levitation of heavy particle-bubble aggregates (J. S. Laskowski & W. Bartoniek, Przeglad Gorniczy, 26:250-255 (1970); French Patent, 1,499,990 (1968); G. A. Gruber and M. E. Kelahan, Column Flotation '88, 191-201 (1988); U.S. Pat. No. 4,822,493 (1989); H. Soto and G. Barbery, Miner. & Metall. Process, Feb., 16-21 (1991)]. A pilot plant flotation column with the upward pulp flow was also tested [W. Aliaga and H. Soto, Trans. IMM, 102: C70-73 (1993)].
Spargers used to disperse air into flotation columns are commonly made from porous materials, or cloth on perforated pipe. While such spargers may provide satisfactory air dispersion in pulps with low-electrolyte concentration, at high electrolyte concentration of saturated brine the conditions are entirely different. It was observed that air dispersion through a porous sparger was poor in saturated brine. Spargers employing mechanical forces (shear flow, turbulence, pressure change, etc.) to premix air and liquid have been recognized to provide much better air dispersion in flotation columns. Bubbles generated in such a way are much finer than those generated by porous materials and their maintenance is much easier. With this type of spargers frother is commonly introduced into the liquid stream supplied to the sparger to further assist in the dispersion of air. In mechanical cells, air bubbles are produced by shearing caused by impellers and a high electrolyte concentration does not affect air dispersion significantly.
There are a few other features which make the potash flotation systems differ from conventional flotation. The difference in density between sylvite particles (1.99 g/cm.sup.3) and saturated brine (1.23 g/cm.sup.3) is small. The upward flow is, therefore, less important in the levitation of coarse sylvite particle-loaded bubbles. Because of the high brine density and viscosity, an upward flow may even significantly increase the entrainment of fine gangue mineral particles in concentrate and decrease concentrate grade. Therefore, the rate of the upward flow has to be carefully regulated to achieve the balance between the levitation of coarse potash particles and the fine gangue entrainment.